Add Array::shrink_to_fit(&mut self)

arrow-array/src/array/boolean_array.rs

@@ -308,6 +308,13 @@ impl Array for BooleanArray {
         self.values.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.values.shrink_to_fit();
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+    }
+
     fn offset(&self) -> usize {
         self.values.offset()
     }

arrow-array/src/array/byte_array.rs

@@ -453,6 +453,14 @@ impl<T: ByteArrayType> Array for GenericByteArray<T> {
         self.value_offsets.len() <= 1
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.value_offsets.shrink_to_fit();
+        self.value_data.shrink_to_fit();
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/byte_view_array.rs

@@ -430,31 +430,31 @@ impl<T: ByteViewType + ?Sized> GenericByteViewArray<T> {
     ///
     /// Before GC:
     /// ```text
-    ///                                        ┌──────┐                 
-    ///                                        │......│                 
-    ///                                        │......│                 
-    /// ┌────────────────────┐       ┌ ─ ─ ─ ▶ │Data1 │   Large buffer  
+    ///                                        ┌──────┐
+    ///                                        │......│
+    ///                                        │......│
+    /// ┌────────────────────┐       ┌ ─ ─ ─ ▶ │Data1 │   Large buffer
     /// │       View 1       │─ ─ ─ ─          │......│  with data that
     /// ├────────────────────┤                 │......│ is not referred
     /// │       View 2       │─ ─ ─ ─ ─ ─ ─ ─▶ │Data2 │ to by View 1 or
-    /// └────────────────────┘                 │......│      View 2     
-    ///                                        │......│                 
-    ///    2 views, refer to                   │......│                 
-    ///   small portions of a                  └──────┘                 
-    ///      large buffer                                               
+    /// └────────────────────┘                 │......│      View 2
+    ///                                        │......│
+    ///    2 views, refer to                   │......│
+    ///   small portions of a                  └──────┘
+    ///      large buffer
     /// ```
-    ///                                                                
+    ///
     /// After GC:
     ///
     /// ```text
     /// ┌────────────────────┐                 ┌─────┐    After gc, only
-    /// │       View 1       │─ ─ ─ ─ ─ ─ ─ ─▶ │Data1│     data that is  
-    /// ├────────────────────┤       ┌ ─ ─ ─ ▶ │Data2│    pointed to by  
-    /// │       View 2       │─ ─ ─ ─          └─────┘     the views is  
-    /// └────────────────────┘                                 left      
-    ///                                                                  
-    ///                                                                  
-    ///         2 views                                                  
+    /// │       View 1       │─ ─ ─ ─ ─ ─ ─ ─▶ │Data1│     data that is
+    /// ├────────────────────┤       ┌ ─ ─ ─ ▶ │Data2│    pointed to by
+    /// │       View 2       │─ ─ ─ ─          └─────┘     the views is
+    /// └────────────────────┘                                 left
+    ///
+    ///
+    ///         2 views
     /// ```
     /// This method will compact the data buffers by recreating the view array and only include the data
     /// that is pointed to by the views.
@@ -575,6 +575,15 @@ impl<T: ByteViewType + ?Sized> Array for GenericByteViewArray<T> {
         self.views.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.views.shrink_to_fit();
+        self.buffers.iter_mut().for_each(|b| b.shrink_to_fit());
+        self.buffers.shrink_to_fit();
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/dictionary_array.rs

@@ -720,6 +720,11 @@ impl<T: ArrowDictionaryKeyType> Array for DictionaryArray<T> {
         self.keys.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.keys.shrink_to_fit();
+        self.values.shrink_to_fit();
+    }
+
     fn offset(&self) -> usize {
         self.keys.offset()
     }

arrow-array/src/array/fixed_size_binary_array.rs

@@ -603,6 +603,13 @@ impl Array for FixedSizeBinaryArray {
         self.len == 0
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.value_data.shrink_to_fit();
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/fixed_size_list_array.rs

@@ -401,6 +401,13 @@ impl Array for FixedSizeListArray {
         self.len == 0
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.values.shrink_to_fit();
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/list_array.rs

@@ -485,6 +485,14 @@ impl<OffsetSize: OffsetSizeTrait> Array for GenericListArray<OffsetSize> {
         self.value_offsets.len() <= 1
     }
 
+    fn shrink_to_fit(&mut self) {
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+        self.values.shrink_to_fit();
+        self.value_offsets.shrink_to_fit();
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/list_view_array.rs

@@ -326,6 +326,15 @@ impl<OffsetSize: OffsetSizeTrait> Array for GenericListViewArray<OffsetSize> {
         self.value_sizes.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+        self.values.shrink_to_fit();
+        self.value_offsets.shrink_to_fit();
+        self.value_sizes.shrink_to_fit();
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/map_array.rs

@@ -372,6 +372,14 @@ impl Array for MapArray {
         self.value_offsets.len() <= 1
     }
 
+    fn shrink_to_fit(&mut self) {
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+        self.entries.shrink_to_fit();
+        self.value_offsets.shrink_to_fit();
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/mod.rs

@@ -167,6 +167,12 @@ pub trait Array: std::fmt::Debug + Send + Sync {
     /// ```
     fn is_empty(&self) -> bool;
 
+    /// Shrinks the capacity of any exclusively owned buffer as much as possible
+    ///
+    /// Shared or externally allocated buffers will be ignored, and
+    /// any buffer offsets will be preserved.
+    fn shrink_to_fit(&mut self) {}
+
     /// Returns the offset into the underlying data used by this array(-slice).
     /// Note that the underlying data can be shared by many arrays.
     /// This defaults to `0`.
@@ -365,6 +371,15 @@ impl Array for ArrayRef {
         self.as_ref().is_empty()
     }
 
+    /// For shared buffers, this is a no-op.
+    fn shrink_to_fit(&mut self) {
+        if let Some(slf) = Arc::get_mut(self) {
+            slf.shrink_to_fit();
+        } else {
+            // We ignore shared buffers.
+        }
+    }
+
     fn offset(&self) -> usize {
         self.as_ref().offset()
     }

arrow-array/src/array/primitive_array.rs

@@ -1152,6 +1152,13 @@ impl<T: ArrowPrimitiveType> Array for PrimitiveArray<T> {
         self.values.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.values.shrink_to_fit();
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/run_array.rs

@@ -330,6 +330,11 @@ impl<T: RunEndIndexType> Array for RunArray<T> {
         self.run_ends.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.run_ends.shrink_to_fit();
+        self.values.shrink_to_fit();
+    }
+
     fn offset(&self) -> usize {
         self.run_ends.offset()
     }

arrow-array/src/array/struct_array.rs

@@ -364,6 +364,13 @@ impl Array for StructArray {
         self.len == 0
     }
 
+    fn shrink_to_fit(&mut self) {
+        if let Some(nulls) = &mut self.nulls {
+            nulls.shrink_to_fit();
+        }
+        self.fields.iter_mut().for_each(|n| n.shrink_to_fit());
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-array/src/array/union_array.rs

@@ -744,6 +744,17 @@ impl Array for UnionArray {
         self.type_ids.is_empty()
     }
 
+    fn shrink_to_fit(&mut self) {
+        self.type_ids.shrink_to_fit();
+        if let Some(offsets) = &mut self.offsets {
+            offsets.shrink_to_fit();
+        }
+        for array in self.fields.iter_mut().flatten() {
+            array.shrink_to_fit();
+        }
+        self.fields.shrink_to_fit();
+    }
+
     fn offset(&self) -> usize {
         0
     }

arrow-buffer/src/buffer/boolean.rs

@@ -114,6 +114,12 @@ impl BooleanBuffer {
         self.len == 0
     }
 
+    /// Free up unused memory.
+    pub fn shrink_to_fit(&mut self) {
+        // TODO(emilk): we could shrink even more in the case where we are a small sub-slice of the full buffer
+        self.buffer.shrink_to_fit();
+    }
+
     /// Returns the boolean value at index `i`.
     ///
     /// # Panics

arrow-buffer/src/buffer/immutable.rs

@@ -167,6 +167,41 @@ impl Buffer {
         self.data.capacity()
     }
 
+    /// Tried to shrink the capacity of the buffer as much as possible, freeing unused memory.
+    ///
+    /// If the buffer is shared, this is a no-op.
+    ///
+    /// If the memory was allocated with a custom allocator, this is a no-op.
+    ///
+    /// If the capacity is already less than or equal to the desired capacity, this is a no-op.
+    ///
+    /// The memory region will be reallocated using `std::alloc::realloc`.
+    pub fn shrink_to_fit(&mut self) {
+        let offset = self.ptr_offset();
+        let is_empty = self.is_empty();
+        let desired_capacity = if is_empty {
+            0
+        } else {
+            // For realloc to work, we cannot free the elements before the offset
+            offset + self.len()
+        };
+        if desired_capacity < self.capacity() {
+            if let Some(bytes) = Arc::get_mut(&mut self.data) {
+                if bytes.try_realloc(desired_capacity).is_ok() {
+                    // Realloc complete - update our pointer into `bytes`:
+                    self.ptr = if is_empty {
+                        bytes.as_ptr()
+                    } else {
+                        // SAFETY: we kept all elements leading up to the offset
+                        unsafe { bytes.as_ptr().add(offset) }
+                    }
+                } else {
+                    // Failure to reallocate is fine; we just failed to free up memory.
+                }
+            }
+        }
+    }
+
     /// Returns whether the buffer is empty.
     #[inline]
     pub fn is_empty(&self) -> bool {
@@ -554,6 +589,34 @@ mod tests {
         assert_eq!(buf2.slice_with_length(2, 1).as_slice(), &[10]);
     }
 
+    #[test]
+    fn test_shrink_to_fit() {
+        let original = Buffer::from(&[0, 1, 2, 3, 4, 5, 6, 7]);
+        assert_eq!(original.as_slice(), &[0, 1, 2, 3, 4, 5, 6, 7]);
+        assert_eq!(original.capacity(), 64);
+
+        let slice = original.slice_with_length(2, 3);
+        drop(original); // Make sure the buffer isn't shared (or shrink_to_fit won't work)
+        assert_eq!(slice.as_slice(), &[2, 3, 4]);
+        assert_eq!(slice.capacity(), 64);
+
+        let mut shrunk = slice;
+        shrunk.shrink_to_fit();
+        assert_eq!(shrunk.as_slice(), &[2, 3, 4]);
+        assert_eq!(shrunk.capacity(), 5); // shrink_to_fit is allowed to keep the elements before the offset
+
+        // Test that we can handle empty slices:
+        let empty_slice = shrunk.slice_with_length(1, 0);
+        drop(shrunk); // Make sure the buffer isn't shared (or shrink_to_fit won't work)
+        assert_eq!(empty_slice.as_slice(), &[]);
+        assert_eq!(empty_slice.capacity(), 5);
+
+        let mut shrunk_empty = empty_slice;
+        shrunk_empty.shrink_to_fit();
+        assert_eq!(shrunk_empty.as_slice(), &[]);
+        assert_eq!(shrunk_empty.capacity(), 1); // NOTE: `Buffer` and `Bytes` doesn't support 0-capacity
+    }
+
     #[test]
     #[should_panic(expected = "the offset of the new Buffer cannot exceed the existing length")]
     fn test_slice_offset_out_of_bound() {

arrow-buffer/src/buffer/null.rs

@@ -130,6 +130,11 @@ impl NullBuffer {
         self.buffer.is_empty()
     }
 
+    /// Free up unused memory.
+    pub fn shrink_to_fit(&mut self) {
+        self.buffer.shrink_to_fit();
+    }
+
     /// Returns the null count for this [`NullBuffer`]
     #[inline]
     pub fn null_count(&self) -> usize {

arrow-buffer/src/buffer/offset.rs

@@ -133,6 +133,11 @@ impl<O: ArrowNativeType> OffsetBuffer<O> {
         Self(out.into())
     }
 
+    /// Free up unused memory.
+    pub fn shrink_to_fit(&mut self) {
+        self.0.shrink_to_fit();
+    }
+
     /// Returns the inner [`ScalarBuffer`]
     pub fn inner(&self) -> &ScalarBuffer<O> {
         &self.0

arrow-buffer/src/buffer/run.rs

@@ -136,6 +136,12 @@ where
         self.len == 0
     }
 
+    /// Free up unused memory.
+    pub fn shrink_to_fit(&mut self) {
+        // TODO(emilk): we could shrink even more in the case where we are a small sub-slice of the full buffer
+        self.run_ends.shrink_to_fit();
+    }
+
     /// Returns the values of this [`RunEndBuffer`] not including any offset
     #[inline]
     pub fn values(&self) -> &[E] {